Method of controlling stress in gallium nitride films deposited on substrates

ABSTRACT

Methods of controlling stress in GaN films deposited on silicon and silicon carbide substrates and the films produced therefrom are disclosed. A typical method comprises providing a substrate and depositing a graded gallium nitride layer on the substrate having a varying composition of a substantially continuous grade from an initial composition to a final composition formed from a supply of at least one precursor in a growth chamber without any interruption in the supply. A typical semiconductor film comprises a substrate and a graded gallium nitride layer deposited on the substrate having a varying composition of a substantially continuous grade from an initial composition to a final composition formed from a supply of at least one precursor in a growth chamber without any interruption in the supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Utility patent applicationSer. No. 09/922,122, filed Aug. 3, 2001, by Hugues Marchand and BrendanJ. Moran, and entitled “METHOD OF CONTROLLING STRESS IN GALLIUM NITRIDEFILMS DEPOSITED ON SUBSTRATES,” which application claims priority under35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No.60/222,837, filed Aug. 4, 2000, by Hugues Marchand and Brendan J. Moran,and entitled “METHOD OF CONTROLLING STRESS IN GAN FILMS DEPOSITED ONSILICON AND SILICON CARBIDE SUBSTRATES,” both of which applications areincorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.N00014-98-1-0401, awarded by the Office of Naval Research. TheGovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nitride films, and particularly methodsto reduce the formation of cracks in gallium nitride films forsemiconductor devices.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by reference numbers enclosed inbrackets, e.g., [x]. A list of these different publications orderedaccording to these reference numbers can be found below at the end ofthe Detailed Description of the Preferred Embodiment. Each of thesepublications is incorporated by reference herein.)

The deposition of GaN films on silicon substrates is difficult becauseof a large thermal expansion coefficient mismatch between the twomaterials. Most deposition techniques involve the deposition of bufferlayers or stress-relief layers with a distinct composition from that ofthe substrate and that of GaN; there is an abrupt composition variationbetween the buffer layer and the GaN layer. These techniques result inGaN films which are under tensile stress at room temperature. Tensilestress favors the formation of macroscopic cracks in the GaN, which aredetrimental to devices fabricated thereon.

GaN and its alloys with InN and AlN are used in visible or UVlight-emitting devices (e.g. blue laser diodes) as well as high-power,high-frequency electronic devices (e.g. field-effect transistors).Because of the lack of GaN substrates, such devices are typicallyfabricated from a thin layer of GaN deposited on a substrate such assapphire (Al₂O₃) or silicon carbide (SiC). Although both substrates areavailable in single-crystal form, their lattice constant is differentthan that of GaN. This lattice mismatch causes extended defects such asdislocations and stacking faults to be generated at the interfacebetween the substrate and the GaN layer as well as into the GaN layeritself. The use of buffer layers such as AlN or low-temperature GaN andthe optimization of deposition conditions typically yields films withapproximately 10⁹ threading dislocations per square centimeter. Morenovel techniques such as lateral epitaxial overgrowth (LEO),“pendeoepitaxy,” and maskless LEO result in lower dislocation densities(as low as 10⁶ cm⁻²).

Although GaN-based devices are currently being mass-produced using bothsapphire and silicon carbide substrates, the use of silicon substratesis expected to bring about further cost reductions as well asimprovement to the capability of those devices. For example, silicon canbe etched using simple chemicals, which allows simple substrate-removaltechniques to be utilized with GaN-based films or devices. Silicon isalso the material on which most of the electronic devices (e.g.microprocessors) have been developed; integrating GaN-based devices withsilicon-based electronic functions would create new types of systems.Silicon is readily available in large wafer sizes with excellent crystalquality at low cost, such that devices grown on silicon may be lessexpensive than equivalent devices grown on sapphire or silicon carbide.Finally, silicon is a better thermal conductor than sapphire.

The growth of GaN on silicon substrates presents similar challenges ason sapphire and silicon carbide. The lattice mismatch between the (001)plane of GaN and the (111) plane of silicon is 17.6%, compared to 16%for sapphire and 3.5% for silicon carbide. The use of a thin AlN bufferhas yielded GaN films on Si(111) with as low as 3×10⁹ threadingdislocations per square centimeter. However, the thermal expansionmismatch of GaN with silicon is +31%, compared to −26% for sapphire and+17% for silicon carbide. (The positive sign indicates a thermalexpansion coefficient larger for GaN than for the substrate.) Assumingfor the sake of demonstration that the GaN film is stress-free at thegrowth temperature (typically 1000 degrees centigrade), a positivethermal expansion mismatch would result in a GaN film under tensilestress after cool-down to room temperature. GaN films exhibit crackingwhen the tensile stress exceeds approximately 400 MPa. Cracks generallyrender devices inoperable due to electrical shorts or open circuits. Ingeneral the stress associated with the lattice mismatch, including anyrelaxation effect that may occur during growth, is referred to as“grown-in stress”. The stress arising from the thermal expansionmismatch when the film is cooled from the growth temperature to roomtemperature is referred to as “thermal stress”. The sum of the grown-instress and thermal stress is the net stress in the film.

Several methods of forming GaN films on silicon substrates have beensuggested. Takeuchi et al. [1] propose a buffer layer composed of atleast aluminum and nitrogen, followed by a (Ga_(x)Al_(1-x))_(1-y)In_(y)Nlayer. Based on technical papers published by the same group (e.g. [2],[3]) the resulting films are under tensile stress, as can be assessed byphotoluminescence spectroscopy measurements. The films exhibit cracking.Extensive work at the University of California, Santa Barbara (UCSB)resulted in significant improvements in crystal quality using thismethod; however the GaN films were always found to be under tensilestress (200-1000 MPa), which usually caused cracking. Takeuchi et al.[4] also propose 3C—SiC as a buffer layer. The resulting GaN films alsoexhibit cracking, which is strong evidence that they are under tensilestress. Yuri et al. [5] propose an extension of this method wherein thesilicon substrate is chemically etched after the deposition of a thinlayer of GaN on the SiC buffer layer, such that subsequent deposition ofGaN is made possible without the tensile stress problems, associatedwith the presence of the silicon substrate. Marx et al. [6] propose theuse of GaAs as an intermediate layer. Shakuda [7] proposes a method offorming GaN-based light-emitting devices on silicon wafers on which asilicon nitride (Si₃N₄) layer has been deposited.

In all the aforementioned techniques, there is a finite composition stepbetween the substrate and the buffer layer as well as between the bufferlayer and the GaN layer. The difference in composition is associatedwith a difference in lattice constants which, in general, means that acertain amount of elastic energy is present in the layers. The elasticenergy is stored in the form of compressive strain if the (unstrained)lattice constant of the top layer is larger than that of the bottomlayer. The elastic energy is maximized if the top layer growspseudomorphically on the bottom layer, that is, if the top layer adoptsthe in-plane lattice constant of the bottom layer. For the cases underdiscussion the amount of elastic energy may exceed the energy requiredto form defects such as islands or dislocations, which reduce the energyof the strained layer. This is especially true if the growth isinterrupted, because in general growth interruptions allow a coherentlystrained layer to evolve into islands. In this case the elastic energystored in the top layer is reduced compared to the pseudomorphic case.

There is a need for methods of reducing the formation of cracks ingallium nitride films for semiconductor devices. Accordingly, there isalso a need for such methods to produce compressive, rather thantensile, stresses in the films. There is further a need for methods toproduce such films on common substrates such as silicon. The presentinvention meets these needs.

SUMMARY OF THE INVENTION

Methods of controlling stress in GaN films deposited on silicon andsilicon carbide substrates and the films produced therefrom aredisclosed. A typical method comprises providing a substrate anddepositing a graded gallium nitride layer on the substrate having avarying composition of a substantially continuous grade from an initialcomposition to a final composition formed from a supply of at least oneprecursor in a growth chamber without any interruption in the supply. Atypical semiconductor film comprises a substrate and a graded galliumnitride layer deposited on the substrate having a varying composition ofa substantially continuous grade from an initial composition to a finalcomposition formed from a supply of at least one precursor in a growthchamber without any interruption in the supply.

The present invention comprises a deposition sequence that results inthe formation of crack-free device-quality GaN layers on siliconsubstrates using metalorganic chemical vapor deposition (MOCVD). The GaNfilms grown using the method of the present invention are undercompressive stress, which eliminates the tendency of GaN to crack. Thedeposition sequence consists of a continuous grade from a material Awhich has a high aluminum composition (e.g. AlN, Al_(0.5)Ga_(0.5)N) to amaterial B which has a low aluminum composition (e.g. GaN,Al_(0.2)Ga_(0.8)N) over a thickness which constitutes a significantfraction (e.g. 20-100%) of the total thickness of the film being grown.The grade can be accomplished by variety of methods, such as (i)changing the vapor pressure of precursors in the growth chamber; (ii)changing other parameters of the growth chamber such as substratetemperature; or (iii) changing the geometry of the growth chamber. Otherelements (e.g. Si, In, As) can also be introduced in the growth chambersuch that intermediate materials other than AlGaN are deposited, as longas the composition variations are not abrupt. Other layers can bedeposited on the graded layer such that electronic devices (e.g.,field-effect transistors) and optoelectronic devices (e.g.,light-emitting diodes) are formed, in accordance with common practice inthe field. Alternatively, additional layers of GaN or AlGaInN alloyswith thickness exceeding five micrometers can also be deposited on thegraded layer as a means of forming a free-standing GaN substrate. Themethod can also be used to control the stress in GaN films grown onsilicon carbide (SiC) substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic cross-sectional view illustrating the structure ofthe layers fabricated according to the general principles of thisinvention;

FIG. 2 is an optical micrograph showing the surface morphology of agraded layer (AlN to GaN) deposited on a Si(111) substrate according tothe present invention;

FIG. 3 is a cross-section view based on a transmission electronmicrograph (TEM) illustrating the microstructure of a graded layer (AlNto GaN) deposited on a Si(111) substrate according to the presentinvention;

FIG. 4 is a plan-view micrograph based on an atomic force microscopy(AFM) scan illustrating the surface morphology of a graded layer (AlN toGaN) deposited on a Si(111) substrate according to the presentinvention;

FIG. 5 is a simplified process flow diagram illustrating the sequence inwhich precursor chemicals are introduced in the growth chamber accordingto one example of the present invention;

FIG. 6 is a schematic cross-sectional view illustrating the layers usedto fabricate a field-effect transistor (FET) device according to oneexample of the present invention;

FIG. 7 is a set of characteristic curves illustrating the performance ofa field-effect transistor (FET) fabricated according to one example ofthe present invention;

FIG. 8 is an optical micrograph showing the surface morphology of agraded layer (AlN to GaN) deposited on a 6H—SiC substrate according tothe present invention; and

FIG. 9 is an optical micrograph showing the surface morphology of a GaNlayer deposited on a 6H—SiC substrate according to a comparative examplein the case where a thin AlN buffer is used instead of a thick gradedlayer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a semiconductor film 100 of the present invention asa schematic cross-sectional view showing the structure of the layersfabricated according to the general principles of this invention. Atypical method producing the film 100 comprises combining the bufferlayer and the GaN layer into a single deposition step to produce asingle graded gallium nitride layer 102 on a substrate 104. During thedeposition the composition is varied continuously between an initialcomposition 106 and a final composition 108, without any interruption inthe supply of precursors 110 to the growth chamber 112. The initialcomposition 106 is that of a material A suitable for a buffer layerwhich wets the substrate, for instance AlN or an AlGaN compound with amoderate to high aluminum fraction (e.g. 20% or more). The finalcomposition 108 is that of a material B such as GaN or AlGaN with lowaluminum fraction (e.g. less than 20%). The thickness 116 over which thecomposition grade 114 takes place is a significant fraction of the totalthickness 118 being deposited, for example 20 to 80% of a onemicrometer-thick film.

The principal feature of the present invention is that the compositionis varied continuously between the initial composition 106 and the finalcomposition 108 without any interruption in precursor 110 supply. Fromongoing materials studies it appears that the lack of interruption inthe growth process prevents the layers with low aluminum content fromdissipating the elastic energy associated with the lattice mismatchbetween material A and material B. Thus a larger amount of compressivestrain is present in the layer structure than is found when using othermethods. In many cases the compressive stress is large enough tocounterbalance the tensile stress induced by the cool-down proceduresuch that the net stress in the epitaxial layers is compressive.Compressively-strained films do not crack, hence preserving theproperties of any device that may have been subsequently deposited andprocessed.

The grade 114 can be accomplished by a variety of methods known to thoseskilled in the art, such as (i) changing the vapor pressure 120 of atleast one precursor 110 among Ga, Al, and N in the growth chamber 112;(ii) changing other parameters 122 of the growth chamber, e.g. totalpressure, substrate temperature, total flow, rate of substrate rotation,temperature of the reactor walls; (iii) changing the geometry of thegrowth chamber 112, e.g., moving the substrate relative to theinjectors, etc.; or (iv) introducing other elements such as Si, In, orAs in the growth chamber 112 such that intermediate-materials other thanAlGaN are deposited, as long as the composition variations are notabrupt. Other layers can be deposited after the GaN layer such thatelectronic devices (e.g. field-effect transistor) and optoelectronicdevices (e.g. light-emitting diodes) are formed.

The mathematical function relating the composition of the growing filmsto the thickness or time can be made to assume any suitable functionalform with the use of proper process controllers. The simplest case isthat for which the composition varies linearly as a function of time; ifthe flow rates are adjusted such that the rate of deposition remainsconstant with time, this method would produce a composition varyinglinearly with thickness, unless segregation effects occur. In othercases, the rate of composition variation could be smaller (or larger) atthe beginning and the end of the grade to further tailor the grown-instress.

A typical embodiment of the grading process uses AlN as the initialcomposition 106 and GaN as the final composition 108. The compositioncan be controlled by changing the partial pressure of the gallium,aluminum, and nitrogen precursors (trimethygallium, trimethylaluminum,and ammonia, respectively). In one embodiment, the substrate 104 isSi(111) and the total thickness 118 of the deposited layer 102 isapproximately one micrometer. The growth temperature was 1050 degreescentigrade.

FIG. 2 shows an example GaN film of the present invention. The netstress in one example was measured to be 270 MPa (compressive) using alaser deflection measurement. Optical measurements (photoluminescence,Raman) were also performed and confirmed this value. The GaN film 102was free of cracks, as shown in FIG. 2. The microstructure of the filmwas of the single-crystal type.

FIG. 3 is a cross-section view based on a transmission electronmicrograph (TEM) illustrating the microstructure of a graded layer (AlNto GaN) deposited on a Si(111) substrate according to the presentinvention. The dislocation density was higher than in state-of-the artfilms at the onset of growth (>10¹¹ cm⁻²), but, because of dislocationannihilation reactions, was low enough (10⁹-10¹⁰ cm⁻²) at the surface ofthe film to enable device demonstrations, and as will become apparentbelow.

FIG. 4 is a plan-view micrograph based on an atomic force microscopy(AFM) scan illustrating the surface morphology of a graded layer (AlN toGaN) deposited on a Si(111) substrate according to the presentinvention. The surface morphology was similar to that of the state ofthe art GaN films grown on sapphire or silicon carbide substrates asindicated by the atomic force microscopy images. When repeating theprocess for a thinner film (˜0.55 μm) the compressive stress wasmeasured to be 400 MPa. Several embodiments of the grading method areavailable using the present invention.

FIG. 5 is a simplified process flow diagram 500 illustrating thesequence in which precursor 110 chemicals are introduced in the growthchamber 112 according to one embodiment of the present invention. In theexample, the grading process begins with the deposition of silicon onthe surface of the heated silicon wafer by use of a suitable siliconprecursor, for example disilane (Si₂H₆) as indicated by the Si gradeline 502. The grade to an aluminum-containing alloy, as indicated by theAl grade line 504, is effected by introducing a controlled amount of asuitable aluminum precursor such as trimethyaluminum (TMAl), thusforming an aluminum silicide. The silicon precursor 110 is thenprogressively removed from the chamber, thus forming a thin film ofaluminum. A nitrogen precursor such as ammonia (NH₃) is progressivelyadded so as to complete the transition to aluminum nitride (shown by theNH₃ grade line 506), after which the sequence continues with theintroduction of a gallium precursor (e.g. trimethylgallium, TMGa) shownby the Ga grade line 508.

In another embodiment of the present invention, the initial composition106 material consists of silicon (deposited on the substrate 104 asdiscussed above) and the final composition 108 material consists of GaN,but only silicon, gallium, and nitrogen precursors 110 are used suchthat the formation of an AlN intermediate layer is avoided. As reportedby Chu et al. [8] the direct deposition of GaN on Si substrates 104usually leads to island formation and highly-defected GaN films.However, in the present invention, the formation of islands is hamperedbecause the deposition is not interrupted. Since the lattice mismatchbetween GaN and Si(111) is only slightly larger than that between AlNand Si(111), this particular deposition sequence leads to the formationof compressively-stressed GaN on Si(111).

FIG. 6 is a schematic cross-sectional view illustrating the layers usedto fabricate a field-effect transistor (FET) device 600 according to oneexample of the present invention. Several such embodiments of thepresent invention exist wherein additional layers 602 are depositedfollowing the formation of the graded layer 102 on a substrate 104 forthe purpose of fabricating specific devices. The example fabricationprocess consists of a thin (˜0.2<x<˜0.5) Al_(x)Ga_(1-x)N or InGaAlNlayer 602 deposited on top of the graded layer 102 which ends with acomposition of GaN. Following usual processing steps, such as electrodeformation, FETs are produced with the present invention havingcharacteristics comparable to state-of-the-art devices fabricated usingother substrates.

FIG. 7 is a set of characteristic curves illustrating the performance ofa field-effect transistor (FET) device fabricated according to oneexample of the present invention. The curves represent the source-draincurrent as a function of source-drain voltage for increasing gate biasin common-source configuration. The saturation current per unit of gatewidth is 525 mA/mm and the transconductance per unit of gate width is100 mS/mm.

In another embodiment of the invention, additional layers 602 of GaN orAlGaInN alloys with thickness exceeding five micrometers can bedeposited on the graded layer 102 as a means of fabricatingfree-standing GaN substrates. The silicon substrate 104 can be removedeither by chemical etching, mechanical polishing, or by any other meansgenerally in use in the field.

The method can also be applied to growth on silicon carbide substrates104. It has been demonstrated by other groups that the stress in GaNfilms grown on silicon carbide using a thin AlN buffer can becompressive for films thinner than approximately 0.7 μm, while filmsthicker than 0.7 μm are generally under tensile stress. [9] One grouphas reported that films grown using 0.3 μm-thick Al_(0.3)Ga_(0.7)Nbuffer layers were under small compressive stress (˜250 MPa). [10] Usingthe same parameters as for the demonstration on Si(111) described above,the present method produced a 0.65 μm-thick GaN film under 950 MPa ofcompressive stress when grown on a 4H—SiC semi-insulating substrate. Thesame process applied to a 1.9 μm-thick film grown on 6H—SiC resulted in815 MPa of compressive stress.

FIGS. 8 and 9 are optical micrographs comparing results of the presentwith those of a conventional process. FIG. 8 shows the surfacemorphology of a graded layer 102 (AlN to GaN) deposited on a 6H—SiCsubstrate according to the present invention. The graded layer 102 GaNfilm of FIG. 8 was free of cracks and exhibited a smooth morphology.FIG. 9 is an optical micrograph showing the surface morphology of a GaNlayer deposited on a 6H—SiC substrate according to a comparative examplein the case where a thin AlN buffer is used instead of a thick gradedlayer. The stress measured for such a film grown using a thin AlN bufferor a thin grade is typically tensile, on the order of 500 MPa; cracksare present in such films, as shown.

CONCLUSION

This concludes the description including the preferred embodiments ofthe present invention. The foregoing description of the preferredembodiment of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Many modificationsand variations are possible in light of the above teaching.

It is intended that the scope of the invention be limited not by thisdetailed description, but rather by the claims appended hereto. Theabove specification, examples and data provide a complete description ofthe use of the invention. Since many embodiments of the invention can bemade without departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended.

REFERENCES

The following references are all incorporated by reference herein:

-   1. T. Takeuchi, H. Amano, I. Akasaki, A. Watanabe, K. Manabe, U.S.    Pat. No. 5,389,571: Method of fabricating a gallium nitride based    semiconductor device with an aluminum and nitrogen containing    intermediate layer.-   2. A. Watanabe, T. Takeuchi, K. Hirosawa, H. Amano, K. Hiramatsu,    and I. Akasaki, J. Cryst. Growth 128, 391-396 (1993): The growth of    single crystalline GaN on a Si substrate using AlN as an    intermediate layer.-   3. S. Chichibu, T. Azuhata, T. Sota, H. Amano, and I. Akasaki, Appl.    Phys. Lett. 70(16), 2085-2087 (1997): Optical properties of    tensile-strained wurtzite GaN epitaxial layers.-   4. Takeuchi, T. et al., J. Cryst. Growth, December 1991, vol. 115,    (no. 1-4): 634-8.-   5. M. Yuri, T. Ueda, T. Baba, U.S. Pat. No. 5,928,421: Method of    forming gallium nitride crystal.-   6. D. Marx, Z. Kawazu, N. Hayafuji, U.S. Pat. No. 5,760,426:    Heteroepitaxial semiconductor device including silicon substrate,    GaAs layer and GaN layer #13.-   7. Y. Shakuda, U.S. Pat. No. 5,838,029: GaN-type light emitting    device formed on a silicon substrate.-   8. T. L. Chu, J. Electrochem. Soc. 118, 1200 (1971): Gallium nitride    films.-   9. N. V. Edwards, M. D. Bremser, R. F. Davis, A. D. Batchelor, S. D.    Yoo, C. F. Karan, and D. E. Aspnes, Appl. Phys. Lett. 73, 2808    (1998): Trends in residual stress for GaN/AlN/6H—SiC    heterostructures.

10. I. P. Nikitina, M. P. Sheglov, Yu. V. Melnik, K. G. Irvine, and V.A. Dimitriev, Diamond and related materials 6, 1524 (1997): Residualstrains in GaN grown on 6H—SiC.

1. A semiconductor structure, comprising: a silicon substrate; and asingle crystal group-III nitride graded layer on the silicon substrate,wherein at room temperature the graded layer has a varying compositionof a continuous grade from an initial composition to a final compositionand a net compressive stress.
 2. The semiconductor structure of claim 1,further comprising a device layer on a side of the single crystalgroup-III nitride graded layer opposite to the silicon substrate.
 3. Thesemiconductor structure of claim 2, wherein the device layer includes agroup-III nitride material and aluminum.
 4. The semiconductor structureof claim 3, wherein the device layer is formed of GaN at a portion ofthe device layer that is distal from the single crystal group-III gradednitride layer.
 5. The semiconductor structure of claim 2, furthercomprising a source, drain and gate.
 6. The semiconductor structure ofclaim 2, wherein a transistor is formed in the device layer.
 7. Thesemiconductor structure of claim 2, wherein a diode is formed in thedevice layer.
 8. The semiconductor structure of claim 2, wherein thedevice layer has a thickness exceeding 5 microns.
 9. The semiconductorstructure of claim 1, wherein the continuous grade varies linearly. 10.The semiconductor structure of claim 1, wherein the continuous gradevaries at a greater rate at a region proximate to the silicon substratethan distal from the substrate.
 11. The semiconductor structure of claim1, wherein the continuous grade varies at a slower rate at a regionproximate to the silicon substrate than distal from the substrate. 12.The semiconductor structure of claim 1, wherein: a surface of thegroup-III nitride graded layer has a dislocation density that is higherin a portion of the graded layer that is proximate to the siliconsubstrate than in a portion of the graded layer that is distal from thesilicon substrate; and the structure further comprises a layer of GaN onan opposite side of the graded layer from the silicon substrate.
 13. Thesemiconductor structure of claim 1, wherein: a surface of the group-IIInitride graded layer has a dislocation density that is between 109-1010cm-2; and the structure further comprises a layer of GaN on an oppositeside of the graded layer from the silicon substrate.
 14. Thesemiconductor structure of claim 1, wherein: the graded layer has atotal thickness of about 1 micron; and the structure further compriseslayer of GaN on an opposite side of the graded gallium nitride layerfrom the silicon substrate.
 15. The semiconductor structure of claim 1,wherein: the graded layer has a total thickness of about 1 micron; aportion of the layer closest to the silicon substrate is AlGaN and aportion of the layer furthest from the silicon substrate has lessaluminum than the portion closest to the silicon substrate; and thegrade is limited to between 20 to 80% of the thickness of the layer. 16.The semiconductor structure of claim 1, wherein the graded layer is freeof cracks.
 17. A semiconductor structure, comprising: a siliconsubstrate; a single crystal group-III nitride graded layer on thesilicon substrate, wherein the graded layer has a varying composition ofa continuous grade from an initial composition to a final compositionand a net compressive stress; and a group-III nitride device layer on aside of the graded layer opposite to the silicon substrate.
 18. Thesemiconductor structure of claim 17, wherein the device layer includesat least a portion that is GaN.
 19. The semiconductor structure of claim17, wherein: the graded layer is free of cracks; and the device layerincludes a plurality of layers and a transistor is formed in theplurality of layers.
 20. The semiconductor structure of claim 17,wherein: the graded layer is free of cracks; and the device layerincludes a plurality of layers and a diode is formed in the plurality oflayers.